Tower Vibration Testing Video
Towers, like all physical objects vibrate at certain frequencies. These frequencies are called "Natural Frequencies". A tuning fork is an often cited example. Another example is a playground swing (although technically a pendulum, the example serves to illustrate the point). It takes very little energy to excite an object at its natural frequency. Once a swing is moving it doesn't take much energy to keep it moving or to increase the arc. In order to keep the swing moving it is necessary to push it at the end of its arc. The "pushes" must occur at just the right time. Obviously, if you were to push the swing while it was still coming toward you, you would slow the swing not keep it moving. Another way of saying this is that the "push" must be in "resonance" with the frequency of the swing.
The blades of a wind turbine are a large heavy rotating mass. If the rotational speed of the blades happens to correspond to the natural frequency of the tower, the tower will begin to vibrate. If the blades were to continue to be in resonance with the tower, the amplitude of the vibrations could increase and, if there were not enough damping in the tower, eventualy damage the structure. It is therefore important to ensure that the natural frequency of the tower does not correspond to the rotational frequency of the turbine rotor. Aerostar tests it towers to ensure that no damaging frequencies exist. Aerostar turbines should only be installed on towers supplied and manufactured by Aerostar.